The present invention relates to a cylinder block for an engine having at least one crank chamber, which cylinder block has a structure for suppressing pressure fluctuation in the crank chamber, thereby reducing pumping loss.
During the operation of an engine, reciprocation of pistons fluctuates the pressure in the crank chamber and the interior of the cylinders, which communicate with the crank chamber. Accordingly, pumping loss occurs.
Conventionally, to reduce pumping loss, a technique has been proposed in which adjacent crank chambers are connected with each other in a crankcase.
Patent documents disclosing such a technique include Japanese Laid-Open Patent Publications No. 2000-136752, No. 2002-180900, No. 2003-74408, No. 2001-241356.
(1) Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-136752 proposes a cylinder block in which through holes are formed in partitions, and the centers of the through holes in the partitions are not aligned.
(2) Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-180900 discloses a technique for connecting adjacent cylinders by forming through holes in partitions, while biasing the through holes relative to the cylinder axes.
(3) Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-74408 discloses a technique for forming a through hole in a cylinder block, which through hole has an axis parallel to the axis of a crankshaft, wherein part of the through hole opens to the inner surface of the cylinder bores.
(4) Patent Document 4: Japanese Laid-Open Patent Publication No. 2001-241356 discloses a technique for forming a through hole extending along an arrangement direction of cylinders, wherein the through hole is inside a wall that is located in a rear section of a cylinder block with respect to the fore-and-aft direction of the vehicle, and the through hole is connected with the crank chamber.
If adjacent crank chambers in a cylinder block are connected with each other, air that is pushed toward one of the crank chambers as the corresponding piston moves flows to the adjacent crank chamber, which suppresses pressure fluctuation in the crank chambers. Accordingly, the pumping loss is reduced.
If adjacent crank chambers in a cylinder block are connected to each other as in the above shown Patent Documents 1 to 3, the structural constraint only allows the through hole to be machined from the outside of the crankcase. Thus, when forming the through hole in a partition, an unnecessary hole that does not contribute to reduce pumping loss is formed in an outer wall of the crankcase.
Since combustion pressure acting on a crankshaft causes stress to be concentrated on areas about through holes, forming of unnecessary through holes as described above is best to be avoided. However, conventional cylinder blocks have no measures for such unnecessary holes.
On the other hand, with recent demands for engines of higher power and better fuel economy, a structure of through holes that effectively reduces pressure fluctuation (pumping loss) has been desired.
However, since the cylinder block disclosed in Patent Document 4 has a structure in which a through hole formed in an outer portion of the wall surrounding the cylinders is connected to the crank chamber through a connector passage, it is possible that, when each piston is reciprocated and pushes air in a section of the cylinder adjacent to the corresponding crank chamber, the air is not smoothly discharged to another crank chamber. In such a case, pressure fluctuation is not sufficiently reduced.